Photofinishing systems customarily prescan a strip of (35 mm) film to examine the quality and color content of the respective image frames prior to transferring the images to print paper. On the basis of this prescan examination, exposure control parameters of the imaging optics components are defined so that, during a subsequent pass of the film, the projection of each image onto the print medium will be properly adjusted to yield an acceptable quality hard copy print.
In order to demarcate where each image is located on the film strip, respective notches are formed along the edge of the film, for example between successive frames, or centrally of each frame. During rescan, these notches are used to identify successive frames and thereby call up previously defined exposure control parameters for adjusting the projection optics.
A shortcoming of such an exposure control procedure is the fact that a notch is sometimes missed. When this happens, there is a mismatch between the current image frame and prescan-derived exposure control parameters. The result is a poor quality set of prints, making it necessary for the photofinisher to reprocess the film strip, which entails additional time, and costs associated with the wasted print materials. Now although a minor lag in processing can be adequately managed, when a large reel of film is to be scanned continuously on a single machine and then printed on the same (or other) machine, it is too complex to track and detect possible sequence errors. As a result, almost no equipment currently available attempts to track such errors over multiple film strips.